In general, concrete has excellent compression strength, durability, and hardness, but has low tensile strength, flexural strength, impact strength, and energy absorption capability, thereby having weak limits under tensile or dynamic loading.
In order to ameliorate such a weakness, a method for reinforcing cement by using a predetermined steel fiber has been widely used, and the size of the steel fiber and a mixing amount of the steel fiber are adjusted depending on the application.
Various types of steel fiber related technologies, such as straight steel fiber having hooked ends and a circular cross-section disclosed in Korean Patent Laid-Open Publication No. 2013-0129385, ring-shaped steel fiber of which opposite ends are disposed apart from each other to improve flexural toughness by reducing a rebound rate when depositing and preventing extraction after depositing, and the like, have been disclosed until now.
As described, a conventional concrete reinforcing steel fiber has been applied to shotcrete, which is a secondary passive reinforcing member, in tunnel excavation, and is commonly applied to a floor slab which requires crack control and has a difficulty in reinforcement using steel bars due to its small cross-section.
Steel fiber applied to the shotcrete has a diameter of 0.5 mm to 0.55 mm and a length of 30 mm to 35 mm, but steel fiber applied to the floor slab usually has a diameter of 0.75 mm to 0.90 mm and a length of 50 mm to 60 mm.
In a case of a conventional hooked steel fiber which is as shown in FIG. 7, opposite ends of a straight main body are generally formed in the shape of a hook by bending the opposite ends at a predetermined angle. This is because, when the steel fiber is pulled out from the concrete due to a tensile force applied thereto after a crack is generated, adhesion performance is deteriorated since there is a significant deterioration of pullout resistance strength in the straight portion, except for the hooked portion. Accordingly, the conventional hooked steel fiber cannot improve mechanical performance.
In order to solve such a problem, various techniques that implement characteristics have been disclosed in Korean Patent No. 1,073,393, Korean Utility Nos. 361,900 and 406, 191, and U.S. Pat. No. 6,060,163.
However, the conventional concrete reinforcing steel fiber, specifically, steel fibers that are designed for improvement adhesion performance, require a relatively short length or high tensile strength so as to improve adhesion performance.
In particular, in case of steel fiber having ring-shaped separated ends like the steel fiber disclosed in Korean Patent Laid-Open Publication No. 1,403,659, the ring-shaped separated ends are disposed higher than a center portion and each end is provided with a closely attached portion that protrudes inwardly and downwardly with an inclination. However, circular-shaped opposite ends need a lapped welding treatment, which may cause quality deterioration or productivity deterioration due to a welding failure.